29 



Wood records that on neutral-red-bile salt lactose agar, the V. P.-|-M. 

 R. strains may develop mucoid colonies which are generally paler than 

 the V. P. M. R.-f- strains. 



Another distinct differentiation may be obtained with the modification 

 of the (Holt-Harris) eosin-methylene-blue agar (see appendix). On this 

 medium the well isolated coli colonies are about three m. m. in diameter 

 appearing very dark, almost black by transmitted light and by reflected 

 light they seem to be button-like, often concentrically ringed with a dis- 

 tinct greenish metallic sheen. Those of aerogenes, on the other hand, are 

 larger, tend to run together, are markedly convex, very much lighter in 

 color, (when viewed by transmitted light), and a metallic sheen is rarely 

 observed. That these cultural differences are quite reliable, at least for 

 routine work, is indicated by a report of Levine, who found that 96.9 per- 

 cent of 122 colonies picked as of the coli section from their appearance 

 on eosin methylene blue agar and 82.4 percent of 102 colonies supposedly 

 of the aerogenes section were confirmed by subsequent tests. 



Correlation of Reactions. The studies of carbohydrate and nitrogen 

 metabolism and cultural characters, all indicate that the colon group em- 

 braces two distinct sections which may be readily distinguished by a num- 

 ber of tests including (1) the gas ratio, (2) acidity to methyl red, (3) the 

 Voges Proskauer reaction, (4) growth in uric acid medium, and (5) ap- 

 pearance of colonies on agar (Endo and eosin-methylene-blue). 



Although no investigator has employed all these reactions simultan- 

 eously, they are known to be very strikingly correlated. Thus Clark and 

 Lubs in 1915 observed a perfect correlation between the gas ratio and the 

 methyl red test. Levine pointed out that the strains which were positive 

 for methyl red were negative for the V. P. test and vice versa. The cor- 

 relation between these two reactions had been extensively confirmed. 

 Johnson; Burton and Rettger; Chen and Rettger; Hulton; Greenfield; and 

 many others have observed an almost perfect correlation. Clark and 

 Lubs (1917) correlated the gas ratio w^ith the V. P. and M. R. reactions, 

 while Koser and Chen and Rettger have shown that the uric acid test, 

 the V. P. and methyl red reaction were strikingly confirmatory of each 

 other. 



Choice of a Routine Differential Test. The question naturally 

 arises as to which test shall be employed or at least given preference in 

 future studies of this group. A choice of a test must of course be de- 

 pendent upon the nature of the work at hand. For investigational studies 

 it should be emphasized that the carefully determined gas ratio, as is urged 

 by Rogers, Clark, etc., is of fundamental importance and that in order 

 to throw further light upon the reliability of the other reactions men- 

 tioned it is desirable that they should all be considered and observed. The 

 problem, however, is quite different when applied to routine water an- 

 alysis. There the time available is limited, the apparatus and other lab- 

 oratory facilities are at a minimum, and the skill of the analyst, we must 

 regrettably admit, is too frequently not comparable with that of the chem- 

 ist. 



